1. Field of the Invention
This invention relates to a flux-cored wire for gas-shielded arc welding, which is improved in working characteristics and capable of producing a weld metal of excellent physical properties, particularly excellent impact properties at low temperature.
2. Description of the Prior Art
Gas-shielded arc welding using a flux-cored wire containing a titania-base flux and a shielding gas such as carbon dioxide gas, argon, helium or the like is widely employed for welding structures of mild steel and high tensile steel of 50 kgf/mm.sup.2 class by reason of excellent bead appearance and superiority in performance quality and efficiency. However, the weld metal which is obtained by the use of a wire with a titania-base flux has a low impact value so that it has been considered difficult to apply such a flux-cored wire to thick plates (e.g., thicker than 25 mm) which require welding of a large heat input, or to steels of higher classes which are governed by severe standards as regards impact value, including the grade III Rules for American Bureau of Shipping (e.g., higher than 5.5 kgf-m at -45.degree. C. to -60.degree. C.).
As disclosed in Applicants' Japanese Patent Publication No. 56-6840, a wire with a titania-base fluxing material containing suitable amounts of Ti and/or TiO.sub.2 and B and/or B.sub.2 O.sub.3 in a filler flux ensures good working characteristics in welding operation and can form a weld metal of good impact value at low temperature, but said metal has a high fracture transition temperature (vTrs: 50% brittle fracture), making it unsuitable as a welding material for the low temperature steels which require a high impact value at -45.degree. C. to -60.degree. C.
On the other hand, with regard to a wire using a flux other than the titania-base flux, it is known, for example, from Laid-Open Japanese Patent Application No. 52-116746 that a wire using a basic flux has a good impact value but is inferior to a wire with a titania-base flux in bead appearance and shape and working characteristics in welding operation (imperfect slag covering and much spattering).